1. Field of the Invention
The present invention relates to a semiconductor device using an oxide semiconductor and a manufacturing method thereof.
In this specification, the “semiconductor device” means any device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and an electronic device are all included in the category of the “semiconductor device”.
2. Description of the Related Art
In recent years, attention has been drawn to a technique for forming a thin film transistor (TFT) by using a semiconductor thin film formed over a substrate having an insulating surface. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices and have been widely utilized as switching elements for image display devices.
Some metal oxides have been known as materials showing semiconductor characteristics. Examples of such metal oxides are tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. A thin film transistor in which such a metal oxide having semiconductor characteristics is used for a channel formation region is known (Patent Documents 1 and 2).
Further, there is a disclosure that zinc oxide, magnesium zinc oxide, or cadmium zinc oxide is used as a semiconductor in Patent Document 3.